Electrolyte for electrolytic capacitor

ABSTRACT

AN ELECTROLYTE FOR ELECTROLYTIC CAPACITIORS WHICH COMPRISES N-METHYLFORMAMIDE AS A SOLVENT AND AT LEAST ONE OF AMMONIUM, AMINE AND ALKALI METAL ACID MALEATE AS A SOLUTE DISSOLVED THEREIN. THE ELECTROLYTE HAS A STABLE LOW VISCOSITY AND A HIGH CONDUCTIVITY SO THAT AN ELECTROLYTIC CAPACITOR USING THE SAME SHOWS A HIGH PRFORMANCE OVER A WIDE RANGE OF TEMPERATURE FOR A LONG PERIOD OF TIME.

United States Patent ()fice 3,812,039 Patented May 21, 1974 US. Cl. 25262.2 2 Claims ABSTRACT OF THE DISCLOSURE An electrolyte for electrolytic capacitors which comprises N-methylformamide as a solvent and at least one of ammonium, amine and alkali metal acid maleate as a solute dissolved therein. The electrolyte has a stable, low viscosity and a high conductivity so that an electrolytic capacitor using the same shows a high performance over a wide range of temperature for a long period of time.

The present invention relates to an electrolyte for electrolytic capacitors showing a high performance over a wide range of temperature for a long period of time.

With respect to dry electrolytic capacitors, it may be fairly said that their performance is largely dependent upon the kind of the electrolyte used therein. As such electrolyte, there are known various and numerous combinations of solvents and solutes dissolved therein, of which examples are as follows: ethylene glycol-boric acid (Japanese Pat. No. 4,5 89/54), ethylene glycol-adipic acid (Japanese Pat. 'No. 14,420/60), dimethylformamideionogen (Japanese Pat. No. 3,984/58), dimethylformamide-water-polyvalent alcohol-ionogen (Japanese Pat. 'No. 3,389/56), etc. Because of the unsatisfactory physico chemical properties of the solvents and the solutes, however, these known electrolytes cannot provide any electrolytic capacitor of a high and stable performance over a wide range of temperature from low to high temperatures. For instance, the electrolytes using ethylene glycol show a relatively high viscosity, particularly at a low temperature, and their conductivity is not suflicient. Further, for instance, the electrolytes with dimethylformamide tend to lose the solvent at a high temperature in view of the fact that the boiling point of dimethylformamide is relatively low, and the life of an electrolytic capacitor at a high temperature is thus not long.

As the result of the study seeking an electrolyte of a stable, low viscosity and a high conductivity, it has now been found that the use of N-methylformamide (hereinafter referred to as NMF) as a solvent and a mono salt of a saturated or unsaturated hydrocarbon dicarboxylic acid having not more than carbon atoms as a solute can provide such electrolyte, and an electrolytic capacitor therewith shows a high performance over a wide range of temperature from low to high temperacures for a long period of time.

NMF has a relatively high boiling point (i.e. 180 to 185 C.) and a relatively low viscosity even at a low temperature (i.e. ca. cp.i. at 60 C.). Because of these favorable properties, attempt has been made to use NMF as a solvent for an electrolyte (Japanese Pat. No. 14,420/68). Namely, NMF was used with ammonium salicylate, ammonium citrate or ammonium borate to make an electrolyte for electrolytic capacitors. However, the solubility of these salts in NMF is not sufficient, and the conductivity of the resulting electrolyte is nnsatis factory. Contrary to such known salts, the mono salt of the saturated or unsaturated hydrocarbon dicarboxylic acid having not more than 10 carbon atoms shows a relatively high solubility in NMF and the electrolyte therewith afi'ords a high conductivity. In this connection, it should be noted that the saturated or unsaturated hydrocarbon dicarboxylic acid is utilizable only when in the form of a mono salt, i.e. an acidic salt. In case of'a di salt, i.e. a neutral salt, the solubility is not sufiicient, and the conductvity of the resultant electrolyte is low.

According to the present invention,-an electrolyte for electrolytic capacitors comprising NMF as a solvent and a mono salt of a saturated or unsaturated hydrocarbon dicarboxylic acid having not more than 10 carbon atoms as a solute dissolved therein is provided.

As the mono salt of the saturated or unsaturated hydrocarbon dicarboxylic acid, there may be used the monoammonium, monoamine or monoalkali metal salt. Specifically, the following mono salts are exemplified: monosodium maleate, monopotassium maleate, monoammonium maleate, monoammonium adipate, monopotassium citraconate, monoammonium citraconate, monoammonium isophthalate, monosodium isophthalate, monopotassium isophthalate, etc. Among them, monoammonium maleate and monoammonium citraconate are particularly preferred.

The concentration of the solute in the solvent is not less than 1% by weight and usually not less than 2% by weight. A closer concentration to the saturation is more favorable. The most preferred is a saturated solution of the solute in the solvent.

Although the electrolyte usually consists of NMF and at least one of the mono salts of the saturated or unsaturated hydrocarbon dicarboxylic acids, there may be incorporated any other solvent or additive insofar asthe electrical property of the electrolyte is not materially diminished.

The electrolyte of the present invention has, as stated above, a low viscosity without any considerable variation on the change of temperature and shows a high conductivity over a wide range of temperature. When it is used for a dry electrolyte capacitor according to a per se conventional manner, the electrolytic capacitor exhibits a high performance.

Still, it may be noted that a mono salt of maleic acid, particularly monoammonium maleate, is generally of high dissociation so that, even when employed with any solvent other than NMF, the resulting electrolyte is more conductive than any conventional one. Thus, a monoammonium, monoamine or monoalkali metal salt of maleic acid may be used in a solvent such as polyvalent alcohols (e.g. ethylene glycol, propylene glycol), ethers (e.g. ethylene glycol monomethyl ether), formamides (e.g. N-methylformamide, N,N-dimethylformamide) or the like in a. concentration of not less than 1% by weight.

Practical and presently preferred embodiments of the present invention are illustratively shown in the following examples.

EXAMPLE 1 Monoammonium maleate (10 g.), prepared by adding two equivalent amounts of maleic acid to a solution of one equivalent amount of ammonia in water, evaporating water from the resultant mixture and drying the resulting product in a desiccator, is dissolved in NMF ml.) at room temperature to make solution, which is utilizable as an electrolyte for an electrolytic capacitor showing a high performance over a wide range of temperature from low to high temperatures as a solid capacitor does.

In the similar manner but replacing monoammonium meleate by any other mono salt of a saturated or unsaturated hydrocarbon dicarboxylic acid, there is obtained the corresponding electrolyte.

The specific resistances of some electrolytes as prepared above are shown in the following Table 1.

TABLE 1 Specific resistance Solute (g.) Solvent (ml.) at 25 C.)

Monoammonium adipate (10).. NMF (100) 250 Monopotassium citraconate (10) NMF (100) 130 .Monoammonitun lsophthalate NMF (100) 250 Monoammonium maleate (10)-- NMF (100) 72 The electric properties of aluminum dry electrolytic capacitors using the electrolytes as prepared above (Sam- .ple A: NMF-j-monoammonium adipate. Sample B:

NMF-i-monopotassium citraconate. Sample C:

NMF+monoammonium isophthalate TABLE 2.-TEMPERATL RE CHARACTERISTICS Particularly, the improvement in the tan 8 inclination at lower temperatures is surprisingly remarkable. As used herein, tangent 6 is the dissipation factor as defined in paragraph 1.8 of RS-205, EIA Standard, page 2, published by the Electronic Industries Association, New York, 195 8.

EXAMPLE 2 Monoammonium meleate (30 g.) is dissolved in ethyl ene glycol (100 ml.) at room temperature to make a solution, which is utilizable as an electrolyte for an electrolytic capacitor. The specific resistance is 120 12cm. C). The spark voltage is 110 v.

In view of the fact that the specific resistance attained by any conventional electrolyte using ethylene glycol as the solvent is ordinarily from 250 to 300 Qcm., the electrolyte in Example 2 may be said to be highly conductive.

The electric properties of aluminum dry electrolytic capacitors using the electrolyte as prepared above (Sample E) and using an electrolyte consisting of ethylene glycol and adipic acid as conventionally employed (Sample F) were comparatively examined with a capacitor element of 33 ,uf.-35 v. rated). The results are shown in the following Tables 5 and 6.

TABLE 5.-TEMPERATURE CHARACTERISTICS Change Change Change ratio of ratio of ratio of capacicapacicapacitime, tance, tanc Change Change Change Electrolyte percent Tan 5 percent Tan 5 percent Tan 6 ratio of Capaciratio of Capaciratio of Capacicapacitance capacitance capacitance 22 7 3 04 0 0. 18 +17. 0 0. 09 tance, loss tance, loss tance. loss 9, 7 0, 65 0 0,12 +7.7 0.08 Sample percent (9 ,uf.) percent (:2 t.) percent (9 pf.) 8. 6 0. 52 0 0. 11 +4. 8 0. 07 9. 1 0. 60 0 0.12 +6. 6 O. 08 E +5. 7 25 0 42 -5. 5 410 5. B 0. 25 0 0.07 +6.5 0. 07 F +13 0 130 -12 750 TABLE 6.ENDURANCE TEST Initial value Value after 500 hrs.

Capaci- Capaci- Capacltance Leakage Capacltance Leakage tance loss current tance loss current Sample (all) (9 i.) (ML/H1111.) (pt) (9 at.) a/min.) Appearance E 37. 5 41 1. 2 41. 3 140 0. 7 Not chan ed. F 36.5 130 1.1 36.6 145 0.6 Do. g

NOTE.--A rated voltage was applied in a thermostatic bath set up at 85 0.

TABLE 3.ENDL RANGE TEST Initial value Value after 500 hrs.

Change ratio of Capacl- Leakage capaci- Leakage tance current tance, current Electrolyte 04f.) Tan 6 a/min.) percent Tan 8 (ML/1111B.)

NOTE.(1) A rated voltage was applied in a thermostatic bath set up at 85 C. (2) Rating of capacitor: 10 v., 220 1.

TABLE 4.SHELF TEST Initial value Value after 120 hrs.

Change ratio 0! Capeei- Leakage capaci- Leakage tance current tance, current Electrolyte 1.) Tan 6 a/nun.) percent Tan 6 a/min.)

292 0. 17 2. 5 2. 4 0. 17 52 310 0. 15 3. 6 --1. 6 0. 14 29. 0 313 0. l1 2. 2 4. 1 0. 11 15. 6 310 0. l3 7. 9 l. 6 0.12 29. 6 323 0. 07 2. 3 2. 2 0. 08 3. 0

own-(1) No voltage was is bath set up at N applied in a thermostat 85 C. (2) Rating of capacitor: 10 v., 220 14f.

From the above results, it can be understood that the electrolytic capacitor with the electrolyte of the invention produces a markedly improved performance over a wide range of temperature from low to high temperatures.

EXAMPLE 3 Monoammonium maleate (16 g.) or monosodiurn maleate (10 g.) is dissolved in dimethylformamide (100 ml.) to make a solution, which is utilizable as an electrolyte for an electrolytic capacitor. The specific resistances and the spark voltages of the electrolyte are shown in Table 7.

TABLE 7 Specific resistance (9 cm.) Spark voltag Electrolyte 0. +30 0. +20 0. 25 C. (v.)

DMF (100 ml.) plus monoammomum maleate (16 45 74 210 100 DMF (100 ml. plus monosodium maleate (10 g.) 98 300 100 TABLE 8.-TEMPERATURE CHARACTERISTICS What is claimed is:

+85 c. +25 C. -25 C. 1. A capacitor electrolyte consisting essentially of N- Change Change Change methylformamide solven and at least 1% by weight of an ratio of Capaciratio of Capaciratio of Capaclpack tame calm? tame capmb tame acid maleate solnte consisting essent ally of equimolar tame, 1058 time, 1055 lance, amounts of maleic acid with ammonia, amine or alkali Sample percent (9 pf.) percent (:2 t.) percent (:2 pi.) t 1 me a E 5.7 0 42 -5.5 410 F 0 750 2. The electrolyte of claim 1 wherein the solute consists E1311: g? 3 Iii ,5; essentially of ammonium acid maleate.

TABLE 9.ENDURANCE TEST Initial value Value after 500 hrs.

Gapacl- Capaci- Capacltance Leakage Capacttance Leakage tance loss current tance loss current Sample 1.) (9 1.1.1.) a/min.) 1.) (S2 pf.) a/min.) Appearance 37.5 41 1.2 41.3 0.7 Not changed. as. 4 130 1. 1 36. e o. 6 Do. 33. e 3e 1. 4 36. 2 90 0. 9 Do. 34.1 38 1. 4 34. 8 42 0. 9 D0.

NOTE.A rated voltage was applied in a thermostatic bath set up at 85 0.

References Cited UNITED STATES PATENTS 3,696,037 10/ 1972 Lagercrantz et a]. 25262.2 3,719,602 3/1973 Anderson et al. 25262.2 3,293,506 12/1966 Chesnot 25262.2 X 3,518,499 6/1970 Shephard et al. 25262.2 X 3,223,639 12/1965 Powers et a1. 25262.2 3,588,625 6/1971 Kihara et a1 25262.2 X

OTHER REFERENCES Weiss et 211.: American Chemical Society, vol. 45, p. 2348, 1923.

Maillols et al.: Chem. Abstracts, vol. 71, p. 17366, 1969.

Buchner: Annalen Der Chemie, vol. 49, p. 689, 1844.

OSCAR R. VERTIZ, Primary Examiner I COOPER, Assistant Examiner US. Cl. X.R. 317230 

